1) Field of the Invention
This invention relates to a process for the production of expanded thermoplastic elastomer products. More specifically, the present invention is concerned with a process for producing the expanded products by using a particular olefinic thermoplastic elastomer and as a blowing agent, carbon dioxide.
Expanded products available in accordance with the present invention are excellent in flexibility, heat resistance and surface appearance and are suitably usable, for example, as interior materials for cars, vehicles and ships or as interior materials for buildings such as houses.
2) Description of the Related Art
Conventional processes known for the production of expanded products of elastomers include a process in which an expanded product of an elastomer is obtained by kneading a vulcanizer and a blowing agent with natural rubber or a synthetic rubber, forming the thus-kneaded compound into a predetermined shape and then heating the thus-formed compound to effect vulcanization and expansion.
To form the aforementioned rubber into the predetermined shape by continuous extrusion, for example, the above-described process however requires, before the continuous extrusion, a step so that additives can be added and kneaded batchwise with rubber to obtain a kneaded melt beforehand. In addition, to facilitate the feeding of the kneaded melt to an extruder, it is also necessary to conduct, before the continuous extrusion, a further step so that the kneaded melt can be formed into a ribbon-like shape in advance. As is understood from the foregoing, processes such as that described above need complex production steps and moreover, require considerable time for the vulcanizing and expanding step, and are hence disadvantageous for industrial production.
As processes free of these problems, processes making use of soft olefin plastics, for example, ethylene-vinyl acetate copolymer and low-density polyethylene are already known. According to these processes making use of such soft olefin plastics, the above-mentioned steps can be omitted.
These processes, however, involve a problem in that a considerable limitation is imposed on the application of resulting expanded products because these soft olefin plastics are basically inferior in heat resistance to rubber.
On the other hand, it is known from Japanese Patent Laid-Open No. 26838/1973, Japanese Patent Laid-Open No. 112967/1979 and the like that as materials showing intermediate properties between soft olefin plastics and vulcanized rubber, for example, partially-crosslinked compositions composed of olefin copolymers and olefin plastics are usable as thermoplastic elastomers.
In these thermoplastic elastomers, however, the olefin plastic components are decomposed when dynamically heat-treated in the presence of a peroxide. When melted, these thermoplastic elastomers are therefore inferior in tension so that the expanding gas tends to escape without being retained inside. Expanded products, even if available, are accompanied by problems in that their expansion ratios are as low as 1.5 times or so and marked surface roughening is observed due to the escape of expanding gas.
With a view to resolving such problems, an expanded olefinic thermoplastic elastomer product, which has an expansion ratio of at least 2 times and is free of surface roughening caused by escape of expanding gas, and a process for producing the expanded product with good productivity through simplified steps are proposed in Japanese Patent Laid-Open No. 143297/1997.
This patent publication describes to the effect that an expanded product is obtained by heating an expandable composition composed of a thermoplastic elastomer, an olefin plastic and a blowing agent. A process making use of azodicarbonamide or sodium hydrogencarbonate as a blowing agent is disclosed. This process is described to be able to produce an expanded olefinic thermoplastic elastomer product which is not available from the invention disclosed in Japanese Patent Laid-Open No. 112967/1979.
However, the expansion ratios of expanded products available from the invention disclosed in Japanese Patent Publication No. 143297/1997 are below 2.9 times, thereby making it difficult to consider that a process capable of producing expanded olefinic thermoplastic elastomer products of various expansion ratios for use in a wide variety of application fields has been provided.
A chemical expanding process such as that employed in the production of the above-described expanded elastomer product generally comprises, as disclosed in the patent publication referred to in the above, mixing raw material with an organic blowing agent of a low molecular weight, which decomposes at a molding temperature to produce gas, and then heating the resulting mixture to a decomposing temperature of the blowing agent or higher to effect expansion molding. According to this process, the production of gas is proportional to the molding temperature, and the decomposition temperature can be easily adjusted by adding an expanding aid or the like. Moreover, this process can obtain expanded products having closed cells.
However, in addition to high production cost for the use of a special blowing agent, these expanded products tend to develop discoloration, offensive odor, food sanitation problems and the like due to decomposition residues of the blowing agent, said decomposition residues remaining in the expanded products. There are other problems including smearing of molding machines caused by a chemical blowing agent and defective molding associated with such smearing.
On the other hand, a gas expanding process is a physical expanding process and comprises melting a resin in a molding machine, feeding an organic compound of a low boiling point such as butane, pentane or dichlorodifluoromethane to the molten resin, kneading the molten resin and the organic compound together, and then releasing the resulting mixture into a low-pressure zone to effect expansion molding. The organic compound of the low melting point, which is employed in this process, has high compatibility with the resin and is hence excellent in solubility and also in foam retention, so that it features the availability of expanded products of high expansion ratios. Nonetheless, such blowing agents are costly and moreover, include potential hazardous problems such as inflammability and toxicity. The also have a potential problem of air pollution. Further, there is a move toward the total ban of Freon-series gases led by dichlorodifluoromethane in view of environmental problems such as destruction of the ozonosphere.
With a view to overcoming such problems of the conventional processes, numerous processes making use of an inert gas such as carbon dioxide gas or nitrogen as a blowing agent, said inert gas being clean and economical, have been proposed.
However, the inert gas has poor solubility in a resin because of its low compatibility with the resin. This makes it difficult to provide expanded products with high expansion ratio. For the production of an expanded product of high expansion ratio, it is necessary to raise the pressure of an inert gas upon its injection under pressure into a molten resin so that the gas can be dissolved at a high content in the resin. Accordingly, it has been difficult to produce expanded products with uniform quality.
Accordingly, there is not believed to be any choice other than coming to a conclusion that use of these conventional chemical expanding processes is accompanied with a significant limitation imposed on the range of expansion ratios of available expanded olefinic thermoplastic elastomer products and also that expanded elastomer products can hardly be produced with uniform quality even when any known production process of expanded products, which makes use of an inert gas, is applied.